Flammability and Explosivity
It is important at this point to briefly discuss what is meant by flammability and explosivity. Materials used in the applications of interest here (refrigeration, foam blowing, solvents, aerosol propulsion, and sterilization) are liquids or gases. In many cases, they are stored in one form and used in another or they are present in both forms during use. When flammable liquids burn, combustion actually occurs in the vapor phase, which is formed above the surface of the liquid by evaporation of the liquid. When flammable gases or vapor from evaporated flammable liquids are allowed to mix with air, the mixture can be explosive. (In this document, I use the terms "vapor" and "gas" as synonymous.) In fact, for the materials of interest here, explosions are just rapid combustion in the gaseous state. Explosions are often termed "deflagrations" if the combustion is relatively slow and as "detonations" if it is extremely fast. Thus "burning," "combustion," "explosion," "deflagration," and "detonation" all involve a rapid oxidation and differ primarily in the rapidity of the process and the results (explosions are often highly destructive). For the materials of interest here, flammability is often determined by introducing the material as a gas or as a vapor from an evaporated liquid into a container with air or oxygen and determining whether deflagration occurs. Thus, throughout this document, I use the term "flammable" to indicate whether combustion can occur without regard for whether the combustion occurs in the vapor phase above the liquid/vapor interface of a liquid or as a deflagration or explosion in a gas/air mixture. Halocarbons
The broad class of halocarbons consists of all molecules containing carbon and one or more of the following halogen atoms: fluorine, chlorine, bromine, and/or iodine. Halocarbons, as the term is used here, may also contain other chemical features such as hydrogen, oxygen, and/or nitrogen atoms; carbon-to-carbon multiple bonds; and aromatic rings.
Due to their generally low toxicities and low or non-existent flammability, one family of halocarbons--the chlorofluorocarbons (CFCs), which contain only carbon, chlorine, and fluorine atoms--has been used for many years in a variety of applications. Refrigerants
Air conditioning, refrigerating, and heat pump appliances transfer heat from one area to another. In vapor compression systems, a chemical or mixture of chemicals, the refrigerant or "working fluid", is compressed in one area (the high-pressure side), where heat is given off, and then allowed to expand in a second area (the low-pressure side), where heat is taken up. In most cases, the working fluid condenses in the high pressure area and then evaporates in the low pressure area. A schematic of a typical refrigeration system is shown in FIG. 1.
CFCs have been the refrigerants of choice in many air conditioning, refrigerating, and heat pump appliances. Thus, CFC-12 (See Halocarbon Nomenclature, Center for Global Environmental Technologies, New Mexico Engineering Research Institute, The University of New Mexico, Albuquerque, N. Mex., Revised September 1997 for a discussion of this "halocarbon number" and other halocarbon nomenclature), also known as R-12 or dichlorodifluoromethane (CCl.sub.2 F.sub.2), has been a widely used medium-pressure refrigerant for commercial and residential refrigeration, medium-pressure centrifugal chillers, and automobile air conditioners. CFC-11 (R-11, trichlorofluoromethane, CCl.sub.3 F) has been widely used in low-pressure centrifugal chillers, and CFC-114 (R-114, 1,2-dichloro-1,1,2,2-tetrafluoroethane, CCIF.sub.2 CCIF.sub.2) is widely used by the U.S. Navy for centrifugal chillers. Other CFCs have also been used as refrigerants, either pure or in mixtures
Foam Blowing Agents
The manufacture of plastic foams for insulation, cushioning, and packaging foams requires the use of gas or volatile liquid blowing agents to create bubbles or cells. CFC-11, CFC-12, CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane, CCl.sub.2 FCCIF.sub.2), and CFC-114 have been used as blowing agents in the manufacture of foam plastic products. In addition to their remarkably low toxicities and lack of flammability, CFCs provide plastic closed-cell foams with excellent insulating ability and generally have good materials compatibility.
Solvents
CFC-113 has been widely used as a solvent in metals, electronic, and precision cleaning and/or decreasing. In this application, in addition to acceptable toxicities and low flammability, rapid evaporation is desired. Rapid evaporation decreases or eliminates energy consumption for drying cleaned parts. All CFCs in common use evaporate rapidly. CFCs and related materials have also been used for dissolution of solutes (dissolving) in many applications including aerosol sprays.
Aerosol Propellants and Sterilants
CFCs have also been used as aerosol propellants though this use is decreasing and is nearly absent in some areas of the world, and a mixture of CFC-12 and ethylene oxide (C.sub.2 H.sub.4 O) is used for gas sterilization of medical equipment and devices. Ethylene oxide is the actual sterilant; CFC-12 is added only to decrease the ethylene oxide flammability. It is estimated that in 1989, 95 percent of all U.S. hospitals used an ethylene oxide/CFC-12 mixture as a sterilant.
Global Environmental Problems
CFCs and many other halocarbons, have come to be recognized as serious global environmental threats due to their ability to cause stratospheric ozone depletion and global warming and their significant atmospheric lifetime. The ozone depletion and global warming impact of chemicals such as these is measured by the ozone depletion potential (ODP) and global warming potential (GWP). ODP and GWP give the relative ability of a chemical to deplete stratospheric ozone or to cause global warming on a per-pound-released basis. ODP and GWP are usually calculated relative to a reference compound (usually CFC-11 for ODP and either CFC-11 or carbon dioxide for GWP) and are usually calculated based on a release at the earth's surface. It is important to note that ODP and GWP values must be calculated by computer models; they cannot be measured. As models, theory, and input parameters change, the calculated values vary. For that reason, many different values of ODP and GWP are often found in the literature for the same compound. Nevertheless, the calculation results are very accurate in predicting which compounds are highly detrimental to ozone depletion or global warming, which are only moderately detrimental, and which have very low or essentially zero impacts.
Despite the wide utility of CFCs, their production has been severely restricted due to concerns about stratospheric ozone depletion. In fact, under the Montreal Protocol, an international treaty enacted in 1987 and amended in 1990, 1992, and 1995, the production of CFCs was phased out in all industrialized nations at the end of 1995. Moreover, the production of certain other halocarbon chemicals has also been halted. Thus, the production of methyl chloroform (1,1,1-trichloroethane, CH.sub.3 CCl.sub.3), which like CFC-113 has been widely employed both as a solvent in cleaning applications and as a foam blowing agent, was also ended at the end of 1995 in industrialized countries.
Replacements and Proposed Replacements for Ozone Depleting Chemicals
Among the earliest chemicals proposed as replacements for CFCs and methyl chloroform were the hydrochlorofluorocarbons (HCFCs). These compounds contain hydrogen in addition to carbon, fluorine, and chlorine. The hydrogen atoms in the HCFCs react with hydroxyl free radicals, which are normal constituents of the earth's atmosphere, and this reaction decreases the atmospheric lifetime of HCFCs relative to CFCs. This decrease in atmospheric lifetime limits the amounts of HCFCs that reach the stratosphere to deplete ozone.
HCFC-22 (chlorodifluoromethane, CHCIF.sub.2) has long been the standard refrigerant for home air conditioners; however, HCFCs are now being promoted as CFC replacements in a number of other applications. For example, HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane, CHCl.sub.2 CF.sub.3) is now widely used as a replacement for CFC-11 in low-pressure centrifugal chillers and, along with HCFC-141b (1,1-dichloro-1-fluoroethane, CH.sub.3 CCl.sub.2 F) and HCFC-124 (2-chloro-1,1,1,2-tetrafluoroethane, CHCIFCF.sub.3), as a foam blowing agent. HCFCs such as a mixture of the two isomers HCFC-225 ca (3,3-dichloro-1,1,1,2,2-pentafluoropropane, CHCl.sub.2 CF.sub.2 CF.sub.3) and HCFC-225 cb (1,3-dichloro-1,1,2,2,3-pentafluoropropane, CHCIFCF.sub.2 CCIF.sub.2) are also being used for cleaning. Unfortunately, the atmospheric destruction process for HCFCs is insufficiently efficient to prevent all of a chemical from reaching the stratosphere. Thus, HCFCs exhibit a low, but significant, ODP. For that reason, HCFCs are scheduled for eventual phaseout under the amended Montreal Protocol.
Much research has gone on to find replacements for the CFCs, HCFCs, and methyl chloroform. Hydrofluorocarbons (HFCs), which contain only hydrogen, fluorine, and carbon, and perfluorocarbons (PFCs or FCs), which contain only fluorine and carbon, are being commercialized as replacement chemicals in a number of applications. Since these materials contain no chlorine, bromine, or iodine (required, in most cases, if a compound is to exhibit significant stratospheric ozone depletion), they have a nominally zero ODP. (Here, I use the word "nominally" since calculations have shown an exceedingly small ODP for some of these materials.) However, HFCs and PFCs have very long atmospheric lifetimes and contribute to global warming. In fact, many PFCs have atmospheric lifetimes of several thousand years, compared with a few years for most HCFCs and a few hundred years for most CFCs. Moreover, a number of HFCs and HCFCs are flammable. When present as gases in volume containing oxygen (for instance, when the refrigerant leaks into a room), explosions can occur. Many new refrigerants--1,1-difluoroethane (HFC-152a, CH.sub.3 CHF.sub.2), difluoromethane (HFC-32, CH.sub.2 F.sub.2),1,1,1-trifluoroethane (HFC-143a, CH.sub.3 CF.sub.3),1,1-dichloro-1-fluoroethane (HCFC-141b, CH.sub.3 CCl.sub.2 F), and 1-chloro-1,1-difluoroethane (HCFC-142b, CH.sub.3 CCIF.sub.2)--are flammable and/or explosive, at least under some conditions. (It should be noted, that HCFC-141b and HCFC-142b are less flammable than the other alternatives listed and, in fact, are stated to be nonflammable in some sources. Both of these compounds do, however, have upper and lower flammability limits and HCFC-142b is classified as flammable in refrigeration standards issued by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, which gives no classification for HCFC-141b). This list of flammable HFC and HCFC refrigerants includes some of the most energy-efficient refrigerants, particularly HFC-152a and HFC-32. Flammable refrigerants are often mixed with nonflammable refrigerants to produce lower flammability or nonflammable refrigerant blends for commercialization, and a large number of such blends are now being marketed. In this document, I use the term "dilution" to indicate all non-chemical (i.e., "physical") means by which a nonflammable component reduces flammability (e.g., dilution of oxygen and heat absorption). Reduction of flammability primarily by dilution is less than satisfactory, however, since a large amount of nonflammable refrigerant must often be added to a flammable refrigerant to obtain a nonflammable blend, and this often produces less than optimal properties.
Many non-halocarbons are being commercialized or seriously considered as CFC replacements. The use of hydrocarbon refrigerants such as propane (CH.sub.3 CH.sub.2 CH.sub.3, R-290), isobutane [CH.sub.3 CH (CH.sub.3)CH.sub.3, R600a], and propylene (CH.sub.2.dbd.CHCH.sub.3, R-1270) is increasing in many parts of the world. These materials are highly energy-efficient and have excellent global environmental properties (low atmospheric lifetimes, low GWPs, and zero ODPs); however, they are also highly flammable, and this has limited their use in many countries and in many applications. In some cases, these hydrocarbons are being blended with nonflammable refrigerants (as is being done for flammable HFCs and HCFCs); however, this often reduces the energy efficiency and increases the global environmental impact. Ammonia (NH.sub.3), a highly energy-efficient refrigerant that has been used for many years, has a zero ODP and a very low atmospheric lifetime and GWP; however, its flammability is coming under increasing scrutiny. Blends of ammonia with HFCs are now being considered to decrease the flammability problem.
The hydrocarbon cyclopentane (C.sub.5 H.sub.10) is now used to blow refrigerator insulating foams in some parts of the world, and hydrocarbons such as n-pentane (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.3), isopentane [(CH.sub.3).sub.2 CHCH.sub.2 CH.sub.3 ], n-butane (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.3), and isobutane have long been used in the production of extruded polystyrene foam sheet products. However, these hydrocarbons are flammable. Other flammable chemicals being considered or being used as blowing agents are HCFC-141b, HFC-152a, 2-chloropropane (CH.sub.3 CHClCH.sub.3), and acetone [CH.sub.3 C(O)CH.sub.3 ]. Conversion from CFC and methyl chloroform to flammable blowing agents will entail significant capital investment to ensure worker safety. There is also concern about the ability of foams blown with flammable blowing agents to meet code requirements and safety standards.
No solvent that is equivalent to CFC-113 and methyl chloroform in toxicity and safety; has a low ODP, GWP, and atmospheric lifetime; and is an effective and easily evaporated cleaner has been identified (Tapscott, R. E., and Skaggs, S. R., Identification of Alternatives to CFC-113 for Solvent Cleaning, NASA White Sands Test Facility, Las Cruces, N. Mex., September 1994). A number of new chemicals--e.g., monochlorotoluenes, benzotrifluorides, volatile methyl siloxanes (VMSs), and terpenes--that are being considered or that are now used as solvents have very good global environmental and solvent properties but are flammable, and this flammability limits their use. There will also be increasing use of flammable chemicals long used as solvents-alcohols, petroleum distillates and other hydrocarbons, ethers, esters, and ketones.
A number of flammable materials are being used or considered for use as aerosol propellants. Among these are the hydrocarbons n-butane (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.3), isobutane [(CH.sub.3).sub.2 CHCH.sub.3 ], and propane (CH.sub.3 CH.sub.2 CH.sub.3); dimethyl ether (CH.sub.3 OCH.sub.3); siloxanes; HFC-152a; HCFC-141b; and HCFC-142b.
Flammable materials have been used in the past as refrigerants, foam blowing agents, cleaning solvents, and aerosol propellants; however, the number of applications and usage are increasing due to the production bans on CFCs. Moreover, as regulations on HFCs and HCFCs increase, there will be increasing pressure to use hydrocarbons, petroleum distillates, ethers, and other highly flammable materials.
Solution to Flammability Problems
The applicability of many of the most useful substitutes or potential substitutes for CFCs in refrigeration, foam blowing, cleaning, aerosol propulsion, and chemical sterilization is severely limited by concerns about flammability. As noted earlier, in some cases, blending of nonflammable materials whose mode of action is primarily dilution with flammable materials has allowed the production of nonflammable or low-flammability products; however, this course of action has been less than satisfactory due to the large amount of nonflammable components needed in many cases. What I claim here is the use of highly effective additives to decrease or eliminate flammability of normally flammable refrigerants, foam blowing agents, cleaning solvents, aerosol propellants, and sterilants. The action of these additives is not due solely to dilution, as in existing and proposed blends with nonflammable components; these additives also act chemically to actually suppress flammability. Such additives can be used in relatively low amounts and, therefore, have a decreased influence on the characteristics of the principal component or components. The mode of action is described immediately below.
Bromine- and iodine-containing compounds disrupt the free-radical chain reactions that maintain combustion. This disruption is a highly effective "chemical" mechanism for fire suppression, as opposed to the primarily "physical" (i.e., "dilution") mechanisms of cooling and smothering provided by nonflammable components used to obtain many nonflammable refrigerant and other blends. Iodides, though useful in direct fire protection technologies, appear to have too high a toxicity and too low a stability for serious consideration as additives in the specific applications discussed here. Bromine-containing compounds, such as the halon fire extinguishing agents, are also highly effective chemical fire suppressants. However, bromine-containing compounds in the specific chemical forms used today as fire extinguishing agents (bromochlorofluoro- and bromofluoroalkanes) have high ODPs because of their long atmospheric lifetimes, and their production has been banned in industrialized nations. Moreover, production of the one (briefly) commercialized bromine-containing halon replacement CHBrF.sub.2 (HBFC-22B1) has now also been banned in industrialized nations under the Montreal Protocol along with all other hydrobromofluorocarbons (HBFCs). In this case, the presence of a hydrogen atom in the molecule (without other features described in the present disclosure) was insufficient to achieve the hoped-for low atmospheric lifetime. In fact, none of the many halon substitute technologies now being commercialized contain bromine due to the concern about their expected high ODP. It should be noted that once they enter the stratosphere, bromine-containing compounds are about 40 times more destructive to stratospheric ozone than are chlorine-containing compounds (Solomon, S., and Albritton, D. L., "Time-Dependent Ozone Depletion Potentials for Short- and Long-Term Forecasts," Nature, Vol. 357, pp. 33--37, 7 May 1992).
There is, however, a solution to the problem of stratospheric ozone depletion by bromine-containing compounds. If chemical features that promote extremely rapid atmospheric removal are incorporated into the compounds, insufficient amounts of the materials will reach the stratosphere to cause significant stratospheric ozone depletion. Thus, the compounds will have exceptionally low ODPs, even though they contain bromine, which is normally a strong ozone depleter. In fact, the resulting short atmospheric lifetimes will also result in low GWPs. Using this concept, I have (1) examined mechanisms for removal of compounds from the atmosphere, (2) determined chemical features that could enhance the various removal processes, and (3) carried out calculations to estimate the atmospheric lifetimes. This three-step process has allowed us to invent several families of bromine-containing halocarbons that have very short atmospheric lifetimes. Moreover, my calculations and estimation methods indicated that these compounds had much shorter atmospheric lifetimes than I had expected and that these very short atmospheric lifetimes resulted in very low estimated ODPs. I then discovered that such compounds can be used as additives to normally flammable materials proposed or used in the following five applications covered by this disclosure:
1. Refrigeration PA0 2. Foam Blowing PA0 3. Cleaning, Degreasing, and Solute Dissolution PA0 4. Aerosol Propulsion PA0 5. Sterilization PA0 1. Bromine-Containing Alkenes PA0 2. Bromine-Containing Alcohols PA0 3. Bromine-Containing Ethers PA0 4. Bromine-Containing Amines PA0 5. Bromine-Containing Carbonyl Compounds PA0 6. Bromine-Containing Aromatics PA0 7. Bromine-Containing Non-Fluorinated Alkanes
Thus, pursuant to the present invention, the following seven groups of compounds having short tropospheric lifetimes and correspondingly low ODPs and GWPs, but also having chemical features (specifically, bromine) that promote effectiveness to reduce flammability have been arrived at. These families are the
Accordingly, it is the object of the present invention to provide bromine-containing additives that act chemically to reduce the flammability of refrigerants, foam blowing agents, cleaning agents, aerosol propellants, and chemical sterilants and that are rapidly destroyed or removed by natural processes in the troposphere. I refer to such additives as "tropodegradable." As a result of the rapid degradation in the troposphere or removal from the troposphere, the additives will have very short atmospheric lifetimes, low ozone depletion potentials, and low global warming potentials. My criterion is that the estimated atmospheric lifetime be on the order of days or weeks, giving ODPs and GWPs that approach zero (probably less than 0.02 ODP) for a ground-level release. Note that I do not consider materials such as HCFCs, HFCs, and HBFCs to be "tropodegradable" as defined here, even though such chemicals are partially destroyed in the troposphere. The destruction processes are relatively inefficient compared to those for the additives claimed here, and HCFCs, HFCs, and HBFCs normally have atmospheric lifetimes of years to hundreds of years.